The present invention generally relates to an apparatus and a method for on-line monitoring of a liquid density and more particularly, relates to an apparatus and a method for on-line, real-time monitoring of a liquid density by a densimeter cylinder that contains a density indicator therein for the continuous monitoring of density by flowing the liquid continuously through the densimeter cylinder.
In the fabrication process for semiconductor devices, a pre-processed semiconductor wafer is frequently polished in order to planarize a top surface of the wafer or to remove excess materials from the surface of the wafer. While apparatus for polishing semiconductor wafers is well known in the art, a chemical-mechanical polishing method has been developed for specific applications on silicon wafers. The chemical-mechanical polishing (CMP) method is named as such because both a chemical reaction between a polishing slurry and a polished surface and a mechanical reaction for removing the debris are involved in the CMP process.
More recently, chemical-mechanical polishing apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front surface or the device-side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is planarized or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A cross-sectional view of a CMP apparatus is shown in FIGS. 1A and 1B. As shown in FIG. 1A, a rotating polishing head 14 which holds a wafer, 10 is pressed onto an oppositely rotating polishing pad 12 mounted on a polishing disc 26 by adhesive means. The polishing pad 12 is pressed against the wafer surface 22 at a predetermined pressure. During polishing, a slurry 24 is dispensed in droplets onto the surface of the polishing pad 12 to effectuate the chemical mechanical removal of materials from the wafer surface 22.
An enlarged cross-sectional representation of the polishing action which results from a combination of chemical and mechanical effects is shown in FIG. 1B. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e., has particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e., larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al3O2) may also be used in the slurry composition.
Since the concentration of particles in the slurry solution plays an important role in the CMP process, it must be carefully monitored before it is dispensed onto the surface of a wafer for performing the CMP process. Conventionally, the density, or the specific gravity of a slurry solution can be determined on a batch basis by a standard densimeter. Samples must be regularly taken from a slurry supply tank and tested for its density to insure it falls within a permissible range. This is a laborious and time consuming process, and is subjected to high probability of human errors. A conventional densimeter 30 is shown in FIG. 1C. The densimeter is constructed by an upright, elongated cylinder 32 fabricated of a substantially transparent material. The cylinder is mounted on a base plate 34 for stability and for forming a fluid-tight container. Inside a cavity 36 of the cylinder 32, a density indicator 40 is provided and submerged in a liquid 38. The density indicator 40 is constructed by a floater portion 42 and a measuring stick 44 which are integrally joined together. The floater portion 42 is constructed of a material that is suitable for the density of the liquid 38 to be measured such that it suspends in the liquid as shown in FIG. 1C. The density of the liquid 38 in the cavity 36 can be read by the position of the marker 46 on the measuring stick relative to the graduated scale 48 on the cylinder wall 50. The process does not allow an on-line, real-time monitoring of liquid density.
Another device which utilizes ultrasonic waves sent through a liquid medium has also been used to measure density of a liquid. The device is very expensive and the technique is operator sensitive and subjected to a number of material parameters which may lead to inaccurate readings. For instance, when the liquid material contains air bubbles, the density reading obtained by the ultrasonic method may be greatly affected. Furthermore, due to the high cost of the ultrasonic equipment, the on-line density measurement technique cannot be used at all fabrication facilities. Moreover, the equipment is a complicated electronic device that requires an elaborate calibration procedure which must be flawlessly performed in order to obtain accurate results.
It is therefore an object of the present invention to provide an apparatus for on-line monitoring a liquid density that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for on-line monitoring of a liquid density that can be used for the continuous monitoring of a liquid density used in a chemical process.
It is a further object of the present invention to provide an apparatus for on-line monitoring of a liquid density which can be carried out on a real-time basis in a chemical process.
It is another further object of the present invention to provide an apparatus for on-line monitoring of a liquid density by utilizing a density indicator situated in a cylindrical-shaped housing for a liquid to flow therethrough continuously such that a real-time monitoring of the liquid density can be obtained.
It is still another object of the present invention to provide an apparatus for on-line monitoring of a liquid density by utilizing an upright, cylindrical housing for flowing a liquid therethrough equipped with a density indicator and positioning guides for holding the indicator in an upright position.
It is yet another object of the present invention to provide an apparatus for on-line monitoring of a liquid density which is equipped with optical sensing means for alerting a machine operator when the liquid density measured is out of specification.
It is still another further object of the present invention to provide a method for on-line monitoring of a liquid density in a chemical process by flowing a liquid through an upright, cylindrical-shaped housing equipped with a density indicator therein for the real-time monitoring of the liquid density.
It is yet another further object of the present invention to provide a method for on-line, real-time monitoring of a liquid density in a chemical process on a continuous basis which includes the use of an automatic detection apparatus which alerts a machine operator of an out of specification density measured.
In accordance with the present invention, an apparatus and a method for on-line, real-time monitoring of a liquid density in a chemical process are provided.
In a preferred embodiment, an apparatus for on-line monitoring of a liquid density can be provided which includes an upright, cylindrical housing that is substantially transparent and fluid-tight defining a cavity therein, a liquid inlet tube in fluid communication with a lower portion of the cavity, a liquid outlet tube in fluid communication with a middle portion of the cavity, a quantity of liquid in the cavity which has a top surface substantially at the same elevation of the liquid outlet tube, a density indicator constructed of a float and a measuring stick integrally joined together such that when the float is submerged in the liquid, the measuring stick is emerged above the top level of the liquid, and at least two positioning guides fixedly attached to the cylindrical housing adapted for guiding the measuring stick in a substantially upright position.
In the apparatus for on-line monitoring of a liquid density, the cylindrical housing may further include a graduated scale marked thereon for determining the relative position of the measuring stick when the float is submerged in a liquid. The apparatus may further include a stabilizing plate positioned at the lower portion of the cavity in the housing with the liquid inlet tube connected therethrough for feeding a liquid downwardly toward a bottom of the cylindrical housing, the stabilizing plate partially separates the lower portion of the cavity from the cavity. The apparatus may further include a sensing device for sensing the position of the measuring stick and for sending out a signal to a process controller on a process machine which consumes the liquid.
In the apparatus for on-line monitoring of a liquid density, the sensing device may include a pair of optical sensors for sensing a predetermined point on the measuring stick inbetween a high density mark and a low density mark. Each of the optical sensors may include an optical beam sender and an optical beam receiver. The quantity of liquid in the cavity may be a slurry solution used in a chemical mechanical polishing process. The apparatus may further include a flow regulating device in the liquid inlet tube for adjusting a flow rate of the liquid flowing through the inlet tube. The at least two positioning guides each has an eyelet for slidingly engaging the measuring stick therethrough.
The present invention is further directed to a method for on-line measuring a liquid density in a chemical process which can be carried out by the operating steps of providing an upright, cylindrical housing in a substantially transparent material defining a fluid cavity therein, connecting a liquid inlet tube in fluid communication with a lower portion of the cavity partially separated from the cavity by a stabilizing plate, connecting a liquid outlet tube in fluid communication with a middle portion of the cavity, positioning a density indicator in the cavity, the density indicator includes a float portion and a measuring stick integrally joined together such that the float portion submerges in a liquid while the measuring stick portion emerges above a liquid level when a liquid is filled in the cavity, mounting at least two positioning guides to an inside wall of the cylindrical housing for sliding engagement with the measuring stick, filling the cavity with the liquid through the liquid inlet tube until a liquid level is maintained substantially at the liquid outlet tube, and reading a density of the liquid from a relative position of the measuring stick of the density indicator.
The method for on-line measuring a liquid density in a chemical process may further include the step of marking a graduated scale on the upright, cylindrical housing for reading a density. The method may further include the steps of mounting a sensing device on the upright, cylindrical housing, and sensing a position of the measuring stick.
In the method for on-line measuring a liquid density in a chemical process, the sensing device may be a pair of optical sensors for sensing a maximum allowable and a minimum allowable density. The method may further include the step of sending an alarm signal to a process controller when the liquid density measured is outside a range between the maximum allowable density and the minimum allowable density. Each of the pair of optical sensors may include an optical beam sender and an optical beam receiver. The method may further include the step of flowing the liquid into the lower portion of the cavity partially partitioned by the stabilizing plate without causing a turbulent flow in the cavity.
In another preferred embodiment, an apparatus for on-line, real-time monitoring a liquid density can be provided which includes a transparent, fluid-tight, cylindrical-shaped housing in an upright position, a cavity for holding a liquid defined by the cylindrical-shaped housing, a liquid inlet tube in fluid communication with a lower portion of the cavity, a liquid outlet tube in fluid communication with a middle portion of the cavity, a quantity of liquid in the cavity which has a top level substantially at the same elevation as the liquid outlet tube, a density indicator constructed of a float and a measuring stick integrally joined together such that when the float is submerged in the liquid, the measuring stick is emerged above the top level of the liquid, at least two positioning guides fixedly attached to the cylindrical housing adapted for guiding the measuring stick in a substantially upright position, and an optical sensor mounted on the cylindrical-shaped housing for sensing a position of the measuring stick.
The apparatus for on-line, real-time monitoring of a liquid density may further include a process controller for receiving a signal indicative of a density reading from the optical sensor and comparing the signal to a pre-set density value. The optical sensor may include two pairs of optical beam senders and optical beam receivers for sensing a maximum allowable density value and a minimum allowable density value. The apparatus may further include a process controller which stops a chemical process when the density reading obtained by the controller is outside an allowable range of the pre-set density values.